Foreign antigen is known to bind to major histocompatibility complex (MHC) molecules in the form of short peptides that are derived from larger polypeptides. The complex of MHC molecule and peptide is the functional unit recognized by T cells during an immune response. Different MHC molecules, while structurally very similar, may show large differences in their abilities to bind to any given peptide. It is known that small differences in a peptide can greatly affect the ability of that peptide to bind to a given MHC molecule and/or cause a T cell response in either a positive or negative fashion. We are studying the kinetics of different peptides binding to various human and mouse HMC molecules. We are using high performance size exclusion chromatography to examine MHCpeptide complex formation and dissociation with fluorescently labeled peptides. We also examine the ability of these in vitro formed complexes to stimulate appropriate T cells. It is essential that we are using the correct peptides for these studies. It is required that we confirm the identity of the peptides we synthesize via mass spectrometry.